Writer Elizabeth Kolbert set out on a global journey to study extinction. She went on a nighttime trek through streams in Panama, where the once-common Panamanian golden frog, the color of a taxi cab, is now extinct in the wild. She swam in the frigid Tyrrhenian Sea off Naples, with scientists studying what happens to marine life when CO2 emissions make oceans more acidic. And she slogged through the sweltering Amazon Rainforest to chart the effects of habitat loss on biodiversity.

The title of Elizabeth Kolbert's new book, "The Sixth Extinction," means this: We are in the midst of another catastrophic epoch. And she explains, while the other mass extinctions, over half a billion years, were caused by geological forces, this one is caused by one species, human beings.

ELIZABETH KOLBERT: There are five previous mass extinctions, the most recent of which was the one about 65 million years ago that killed off the dinosaurs. And there's a pretty broad consensus that that was caused by an asteroid impact. And so now you'll hear scientists say, we humans are the asteroid.

BLOCK: We are the asteroid because, what are they pointing to?

KOLBERT: Well, they're pointing to this idea that we're changing the world very, very rapidly. Maybe not quite as rapidly as an asteroid but, as you look through geological time, rapid change is very unusual. So among the ways that we are changing the world very rapidly is we're changing the atmosphere by pouring a lot of CO2 into it very, very quickly. That also has the impact of changing the chemistry of the oceans very quickly, because a lot of our carbon emissions end up in the oceans.

We are moving species around the world very fast. When you think about it, it's very, very hard for most organisms to, you know, cross an ocean. Or if they're a marine organism, cross land. That now happens all the time. We're just altering the surface of the Earth; something like 50 percent of the surface of the Earth has been altered by people by now. So those are just some of the ways that we are changing the world very, very quickly.

BLOCK: And when you look at species lost, what do the numbers show is going on right now?

KOLBERT: Well, if you look at the number of species, for example, that are on what's known as the Red List - which is kept by a group that sort of keeps the list of endangered species - they are increasing all the time. It's estimated that a quarter of all mammals on the planet are endangered. It's estimated that about a quarter of all sharks and rays are endangered. A third of all reef-building corals are endangered. Something like 40 percent of all amphibians are endangered. So these numbers are very, very high.

BLOCK: You profile a number of these species through the course of your book. And you go to the southernmost tip of the Great Barrier Reef to look at coral, try to see what's going on with coral reefs. You're about 50 miles off the coast of Australia. How dire was the picture that was painted for you when you went there?

KOLBERT: Well, that was an amazing spot. We were on this tiny, little island that just pokes above the reef. So you're almost effectively living on the reef. And every day these scientists would go out and walk across the reef to go collect water samples. And they were trying to look at how the rate basically at which reefs putting on weight. Because, as one scientist put it to me, they have to keep growing just to keep constant.

And what we're doing by changing the chemistry of the water is we're making their job harder and harder and harder. And the Great Barrier Reef has lost something like 50 percent of its coral cover just over the last 30 years, which is a pretty astonishing figure. And there are very robust predictions in the scientific literature that by about 2050, the Great Barrier Reef is going to have a very, very hard time surviving.

BLOCK: You do have an amazing description in there. You happen to be there at the time of the annual mass spawning of coral. You call it synchronized group sex. And you go out for a nighttime snorkel to see what that's like. Can you describe it?

KOLBERT: What happens is that corals, once a year, they engage in this sort of orgy, I guess you could call it, where they all give up their gametes at one time. And these are these little, tiny bead-like things that contain both sperm and eggs, which is pretty unusual. And so, when they're ready to spawn, they spawn on the same night. This is so they can, you know, mix up their genes. It happens around a full-Moon.

It can be anticipated when it's going to happen. So that's how I knew to be there. And then we went out at night and it looks like it's snowing upwards. All of the coral suddenly start to release these little beads which float to the top, so it's sort of like a snow globe but going in reverse. And it's a wonderful, wonderful sight.

BLOCK: Lets talk more about the human role in this current mass extinction, because you have a line in your book where you say when you think about why humans are so dangerous, maybe you picture a poacher in Africa in 1947 or a logger in the Amazon gripping an ax. But then you say picture yourself holding a book in your lap. In other words, man's impact on nature and on species isn't necessarily malevolent by design.

It's part of what we're doing to the Earth by our very nature of being on the Earth that you're saying is implicated here.

KOLBERT: Yes, I think that a lot of these conversations are often as if there are good people, you know, and bad people. And my point is that change is really what is significant. And many of the qualities that are best about humans, most wonderful, most marvelous, most miraculous: our intelligence, our ability to cooperate and build these tremendous societies, our inventiveness, they have this interesting side-effect, which is that we don't wait for evolution to provide us with a new tool. We make that tool. And that just turns out to be when you're another creature who does have two proceed at a pace of evolution, to be very difficult to deal with.

BLOCK: Well, in the midst of all of the extinction that you're documenting in your book, you also profiled the people who are working as hard as they can to preserve what's left of species that are disappearing. And I was really interested to read about an institute for conservation research near San Diego, where they are taking care of a Hawaiian crow. The crows are extinct in the wild and this particular crow is named Nahoa(ph). And there's one woman who ministers to him in a very particular way. I guess that's for lack of a better word - I'll say she ministers him.

KOLBERT: Yes, she strokes him very lovingly, hoping that a male bird will find this, you know, sexually exciting. How's that? Let's put it that way, so that she can collect a vial of his semen and rush it to Maui.

BLOCK: And how has that been going?

KOLBERT: It hasn't been going well. She - during mating season, she tries to emulate when his mating season on Maui. So it's in the spring. She will spend hours during the several times a week. But when I was there, which is about nine months ago or so, she put it to me: He had still failed to produce any high-quality material.

BLOCK: And when you talked with this woman, who's so dedicated to this cause - her name is Barbara Durrant - what did she tell you about why she does what she does?

KOLBERT: Well, this is an institute a really devoted to trying to save some of our most endangered animals. And I guess I would say it's an act of love. I mean, it's an act of trying the best we can to preserve the remnant of this species in some way. And the really interesting and moving thing I found out about Nahoa, is he seems to have a lot of self-consciousness. And he - crows can imitate human speech. And Nahoa says, I know.

So it sounds a little bit demented, it's hard to understand. But that is the message that he gives you, I know.

BLOCK: You've covered the environment for a long time, written a lot about this. Were you surprised as you read more and studied more and traveled more for this book, surprised at what you found?

KOLBERT: Well, I think I was surprised by the scope and reach of what's going on. I recently, for example, just heard some very eminent scientists say, you know, this may be the last century that we see large animals out in the wild. We're just not going to see them anymore. They're going, to the extent that they exist at all, they will only be in zoos. That's a pretty sobering fact and even I, after researching this book, was pretty sobered to hear them.

BLOCK: Elizabeth Kolbert, her book is titled "The Sixth Extinction: An Unnatural History." Elizabeth, thanks so much.